In what ways is "Educating Rita" by Willy Russell effective as a play?

'Educating Rita" is dramatically effective in many ways. It is also recognised as an exceptional play; it was voted "Best comedy of the year" when performed by the Royal Shakespeare Company in 1980, and just three years after release, it had become the fourth most popular play in Britain. There are obviously factors which make it so effective, and I intend to explore these factors.
Throughout the play, there are only two characters: this is known as a two-hander. Other characters are only mentioned in the play, but in the film have been cast as parts. It worked well for the film, but…